1. Field of the Invention
The present invention relates to a frequency-hopping carrier generator, particularly to a filterless frequency-hopping carrier generator, which generates varied frequencies to reduce the EMI (Electro_Magnetic Interference) peak value according to a first current and a second current.
2. Description of the Related Art
The class-D amplifier (digital audio power amplifier) is an audio power amplifier converting an analog audio signal or a PCM (Pulse Code Modulation) signal into a PWM (Pulse Width Modulation) signal or a PDM (Pulse Density Modulation) signal and then using the PWM or PDM signal to control the switching of a high-power switch device. Therefore, the class-D amplifier is also called a switch power amplifier. The class-D amplifier has a significant advantage: high efficiency. A class-D or digital amplifier comprises an input-signal processing circuit, a switching-signal generating circuit, a switch power circuit (half-bridge or full-bridge) and a low-pass LC filter. The class-D amplifier utilizes a very-high-frequency switching circuit to process audio signals and has the following characteristics:    1. Very high efficiency, usually above 85%; and    2. Compactness (saving much more space than an analog amplifier circuit); and    3. No cracking noise.
Conventional class-D amplifiers have a disadvantage of having to use a low-pass LC filter to reduce EMI, which not only increases the cost, but the bulkiness of a low-pass filter also impairs circuit design. Therefore, advanced manufacturers proposed filterless class-D power amplifiers. In a U.S. Pat. No. 6,545,533, Texas Instruments Incorporated proposed a filterless class-D amplifier using a pseudo noise generator. However, the pseudo noise generator does not synchronize with the PWM carrier of the Class-D amplifier. Thus, the beat frequency of the two signals will interfere with audio signals. As the signal of a pseudo noise generator is a random signal, the beat frequency interference signal thereof is also a random interference signal. Therefore, the interference cannot be reduced with a specific circuit.
In a U.S. Pat. No. 6,847,257, Maxim Incorporated also proposed a filterless class-D amplifier, which has an FFM (Fixed Frequency Modulation) mode and an SSM (Spread Spectrum Modulation) mode, in which a common-mode pulse is added into the original signal. The SSM mode can reduce EMI radiation generated by frequency modulation. However the frequency of the pulse width modulating is varied, cycle to cycle, on a pseudo random basis according to U.S. Pat. No. 6,847,257 claim 11 in Column 7.
Refer to FIG. 1 for a sample spectrogram of a conventional class-D amplifier; wherein the energy peak is −4 db, and the energy level at 5th harmonics is −20 db. Thus, the present invention proposes a frequency-hopping carrier generator, wherein the frequency modulation method is distinct from those mentioned above, to reduce EMI interference when LC filter is not in use. The improvement relative to FIG. 1 with present invention will be discussed in the Detailed Description of the Invention.